The fuel and lubrication oil used in an internal combustion engine contain sulfur, therefore the exhaust gas contains SOx. In this regard, this SOx acts to greatly lower the performance or durability of the exhaust gas purification catalyst or other post-treatment device arranged in an engine exhaust passage, therefore the SOx in the exhaust gas is preferably removed.
Therefore, there is known an internal combustion engine in which an SOx trap catalyst able to trap the SOx contained in the exhaust gas is arranged in an engine exhaust passage (see Japanese Patent Publication (A) No. 2005-133610). This SOx trap catalyst has a property of trapping the SOx contained in the exhaust gas when the air-fuel ratio of the exhaust gas flowing into the SOx trap catalyst is lean and allowing the trapped SOx to gradually diffuse inside the SOx trap catalyst when the temperature of the SOx trap catalyst rises under a lean air-fuel ratio of the exhaust gas and as a result allowing the SOx trap rate to be restored. Therefore, this internal combustion engine is provided with estimating means for estimating the SOx trap rate of the SOx trap catalyst and, when the SOx trap rate falls below a predetermined rate, the temperature of the SOx trap catalyst is raised under a lean air-fuel ratio of the exhaust gas to thereby restore the SOx trap rate.
In this regard, in this SOx trap catalyst, the SOx is trapped and deposits in order from the upstream side, therefore the SOx trapping ability will fall in order from the upstream-side. In this regard, in this SOx trap catalyst, if the SOx trapping ability at the upstream side falls, even if there is a sufficient SOx trapping ability at the downstream side, there are cases where the SOx will end up passing through the SOx trap catalyst. If SOx passes through the SOx trap catalyst in such a way, regardless of whether there is a sufficient SOx trapping ability remaining in the SOx trap catalyst, the SOx trap rate will be judged to have fallen, that is, the SOx trap catalyst will be judged to have become unable to withstand use. In this case, in the aforementioned internal combustion engine, restoration of the SOx trap rate is performed.
However, in this case, if the sufficient SOx trapping ability remaining in the SOx trap catalyst is applied, the period until the SOx trap catalyst becomes unable to withstand use may be extended. That is, the lifetime of the SOx trap catalyst may be prolonged.